Zoned pressure screen

ABSTRACT

Pressure screening apparatus for screening a suspension of papermaking stock employing an enclosing housing and a cylindrical screen, with a drum type rotor, has the screen divided or formed with axially spaced annular screening zones and a corresponding number of annular reject collection areas, one for each screening zone. The collection areas are positioned so as to receive rejects from one of the screening zones only, while a rotor is formed with outlet openings that apply stock suspension to selected ones of the screening zones so that there is no mixing of rejects between screening zones, and each screening zone receives stock suspension for screening independently of the other screening zones.

This invention relates to pressure screens and more particularly to ascreen for removing contaminants from a suspension of paper making pulp.

Closed pressure screens in which a flat or cylindrical screen is used toseparate a suspension of paper-making pulp into an accepts pulp fractionand a reject fraction have long been used for paper pulp cleaning. Suchpressure screens commonly employ a generally cylindrical perforatedscreening member, which may form an aperture pattern made up of eitherholes or slots. A rotating impulse member is positioned to operateadjacent a surface of the screen, which is commonly, but not always, aninner inlet surface, to maintain the stock suspension in a state ofagitation and to provide pressure impulses by which the screeningfunction is aided. The rotating-member may comprise a drum-type rotor inwhich protuberances or foil-shaped sections are mounted on the outersurface and move adjacent to a screen surface, or foils may be mountedon generally radially extending arms for rotation adjacent the screensurface.

Commonly, the stock suspension to be screened is brought in at oradjacent an axial end of a cylindrical screen and, during screening,moves axially along the inlet surface, as stated above, commonly theinner surface of the screen cylinder. At the same time, a rejectsfraction is created by the inhibiting or screening out of undesirablematerial which does not pass through the apertures or openings in thescreen, and this undesirable material also moves axially along thescreen surface until it reaches the end of the screen axially oppositethe inlet end, where it is directed to a rejects accumulation chamberand to a rejects outlet. Examples of pressure screens made and operatedaccording to this principle may be seen in the following patents whichare assigned to the same assignee as this application: Chupka et al.,U.S. Pat. No. 4,328,096 issued May 4, 1986, Martin et al., U.S. Pat. No.4,851,111 issued Jul. 25, 1989, and Weber, U.S. Pat. No. 4,166,028issued Aug. 28, 1979.

With the increase in the quantity of contaminants in waste paper, andthe need to remove such unwanted material from a pulp furnish, it hasbecome necessary to improve the efficiency of the pressure screen inorder to remove more of the contaminants and unwanted material and toincrease the capacity. Screening efficiency may be defined as thepercentage of contaminant removed from the stream of the feed suspensionby the screening operation.

Conventionally, the stock suspension enters at one end of the screen orenters at the center of the screen and flows in opposite directions overthe screen. The multiple foils perform the well known impulse andscreening function such that the fibers are accepted through theperforated or slotted screen while the larger or longer material whichis unable to go through such perforations is retained within thescreening zone. It is also known that such screens cause the stockconsistency to increase between the feed end of the screening zone andthe reject end of the zone while, at the same time, the contaminantconcentration increases as it gathers and flows along the screen inletsurface toward the reject end.

Two principle factors reduce the ability of any screen to retaincontaminants. These are: 1) higher velocity of stock through the screenapertures causes more of the contaminants to flow through the apertures;2) higher concentrations of the contaminants within the screening zoneincreases the probability that contaminants will flow through the screenapertures.

The conventional screening apparatus has a gradient of velocity of thestock through the apertures over the length of the screening zone, thisin an axial length in the case of a cylindrical screen. The highestvelocity is at or adjacent the point of entry of the stock suspensioninto the screening zone where the consistency is the lowest and thecontaminants concentration is also the lowest. On the other hand, thelowest stock velocity is at the reject end of the screening zone wherethe consistency is the highest, due to liquid removal through thescreen, and at the same time the concentration of contaminants is at itshighest level.

Center flow screens with top and bottom reject outlets reduce, but donot eliminate the problem. Rather, center flow screens conventionallyand commonly permit the employment of axially longer or lengthenedcylindrical screens, or two individual screens, one mounted above theother.

It is known that higher efficiency may theoretically be maintained byreducing the axial extent of the screening zones and thus creating aplurality of separate screening zones, and then removing the contaminantfraction associated with each zone. This is the approach suggested inLjokkoi, U.S. Pat. No. 4,915,822 issued Apr. 10, 1990. In patent '822,it is suggested that concentric screens may be divided into a pluralityof axially separate screening zones and the contaminants may be removedthrough an elaborate arrangement of conduits formed in or as part of adrum-type rotor. It is believed, due to the complexity, that thearrangements suggested in '822 have not been commercialized by reason ofthe technical difficulty of employing concentric screens, as shown, withhollow rotor foils therebetween forming conduits for the contaminants.

There is accordingly a need for optimizing the performance of relativelyconventional screen cylinders using relatively conventional impulseinducing rotor apparatus to control contaminant build up and consistencyincrease so that a greater portion of the screen surface is continuallyoperated at or near its most efficient condition.

In the realization of this invention, a screen means, which may be asingle screen or a plurality of separate screens, is divided into aplurality of axially spaced screening bands or zones, and means areprovided for applying the stock suspension under pressure directly tothe inlet side of the screening surface, at each such zones. Suchaxially disposed zones individually form a minor portion of the totalaxial extent of the screening means.

At least one rejects receiving or collection area is provided for eachsuch zone. Preferably a rejects collection region is provided at oneaxial end of each zone, so that each zone is terminated at one end at arejects collection region. At least one such region or collection areais located between a pair of axially spaced screening zones. The totaleffective axial height or length of any screening zone is reduced bydividing the effective screening length into a plurality of individualscreening zones of more limited length, in which the adverse effects ofthe increase in contaminant concentration and in consistency arereduced, and in which the through the screening zone put velocity ismaintained constant over the axial length of the usual high peakvelocity adjacent to the stock entry is eliminated.

The individual screening zones or bands have outlet sides or surfaceswhich open into individual or common accepts collection areas leading toan accepts outlet. The rejects collection areas associated with each ofthe zones, likewise may be connected to a common rejects outlet, or toindividual reject outlets.

In one preferred embodiment of the invention, a cylindrically continuousscreen member is employed in which a plurality of discrete screeningzones are established or defined by the position of annular ringscarried on the outer surface of a drum-type rotor, so that one zone isseparated from the other. In this preferred embodiment, the inletsuspension is directed into the rotor interior and exits throughopenings formed in the rotor surface in immediate proximity to ascreening zone. One or more rejects accumulation regions or areas aredefined in the screen, positioned axially between screening zones, intowhich screen-rejected material may flow. Means are provided, by way ofopenings through the screen wall, by which the reject material isextracted from the rejects accumulation areas.

In another preferred embodiment, two or more generally cylindricalscreens are mounted in axially spaced relation and held together bybolts and spacers. An intervening axial space between the screens formsa rejects collection annulus for one of the screens. In eitherembodiment, the rotor may be internally configured with internal bafflesto form discrete flow passages leading to each of the screening zones.

The invention may be described as a pressure screen for screening asuspension of paper-making stock having an enclosing housing with screencylinder means in the housing, with an inlet surface and an outletsurface. A rotor is mounted for rotation within the screen means and haspressure impulse members or foils mounted for rotation with the rotor,in close proximity to the screen inlet surface. A stock inlet in thehousing directs a stock suspension to be screened into the interior ofthe screen means. A stock outlet in the housing receives screened fiberspassing through the screen means, and a reject outlet receives rejectmaterial from the inlet surface. The improvement comprises or ischaracterized by the fact that the screen means is formed with at leasttwo axially spaced screening zones, and at least one rejects collectionarea is positioned between the screening zones, and in which the inletsurface of one of the screening zones communicates with the rejectscollection area for delivering rejects from the collection zone to arejects outlet. In the case of a pair of cylindrical screen sections orindividual screens, one above the other, the screen zone closer to theinlet may feed the intermediate rejects collection area while the screenzone positioned more remote from the inlet may feed into a conventionalrejects collection area at its remote end.

In another aspect of the invention, the invention may be described as apressure screen for screening a suspension of paper making stock, asoutlined above, in which a drum-type rotor is mounted for rotationwithin the screen and in which the stock inlet in the housing directsthe stock suspension into the interior of the rotor. Such pressurescreen is characterized by a plurality of axially spaced annular zonesincluding at least two axially spaced screening zones and at least oneintermediate rejects collection zone or area. The zones are defined bydiscrete ribs between the screen and the rotor whereby stock to bescreened is directed into the annular screening zones. Openings areformed in the rotor to coincide with the screening zones for deliveringstock to be screened from the inlet directly into the screening zones. Arejects collection passage is formed in the screen leading from thereject area and communicating with a rejects outlet, with a result thatscreening is restricted to the plurality of screening zones in which areject area is positioned intermediate the screening zones.

It is accordingly an important object of this invention to provide animproved and more efficient pressure screen for screening a suspensionof paper making stock, in which a screen is formed by or divided into aplurality of discreet screening zones with at least one rejectscollection area being positioned between a pair of such screening zones.

A further object of the invention is the provision of an improveddrum-type pressure screen for screening a suspension of paper makingstock in which a cylindrical screen is formed or forms a plurality ofscreening zones and in which a drum rotor is provided with outletopenings for delivering a stock suspension to be screened directly intoone of the discrete screening zones.

Another object is the provision of a pressure screen, as outlined above,in which a screen cylinder has formed thereon two or more annularscreening zones and axially separated by a reject collection areapositioned between a pair of screening zones.

In order that the present invention may be more readily understood,reference will now be made to the accompanying drawings, in which:

FIG. 1 is a sectional and partially diagrammatic view of a zonedpressure screen in accordance with one embodiment of this invention;

FIG. 2 is a side view of the cylindrical screen for use in the pressurescreen of FIG. 1;

FIG. 3 is a view, partially in section, of a further embodiment of azoned pressure screen in accordance with this invention;

FIG. 4 is a perspective illustration of the shell of a drum-type rotorincluding internal walls or partitions for the division of incomingstock suspension, for a two-zone pressure screen; and

FIG. 5 is a view similar to FIG. 4 showing the shell of a drum-typerotor with internal walls for the division of incoming flow for athree-zone screen according to this invention.

Referring to the figures of the drawing, which illustrate preferredembodiments of the invention, a pressure screen is illustrated generallyat 10 in FIG. 1 as having an inlet 12 for a suspension of papermakingpulp to be screened. The pulp to be screened is brought into theinterior shell of a closed top drum-type rotor 15, having externalimpulse foils 16 on the outer surface. The rotor 15 is enclosed withinthe housing of the pressure screen, and coacts with cylinder screenmeans in the housing, shown at 20 in FIGS. 1 and 2.

The screen means is an integrated screen cylinder 20 and is divided intoa plurality of axially spaced annular screening zones 22. In thisinstance, three such zones are shown including an upper zone 22a, acentral zone 22b, and a lower zone 22c. The screen cylinder portions inthe zones 22 are formed with conventional screen openings such as in theform of holes or slots.

The drum rotor individual foils 16 are positioned to move in closeproximity to the inner inlet surface of the screen 20 at each of thezones 22a, 22b and 22c. The foils 16 are designated by the referencenumeral 16a, 16b and 16c corresponding to the zone at which theyoperate. The angular positions of the foils 16 in each of the zones arestaggered with respect to the positions of the foils in the other zones,to provide a balanced operation.

The screen 20 is ribbed to define a pair of intermediate rejectscollection areas positioned between a pair of axially spaced annularscreening zones. Thus, the screen 20 has an annular rejects collectionarea 24a which is positioned axially between screening zones 22a and22b, and has a second annular rejects collection area 24b which ispositioned between annular screening zones 22b and 22c. These rejectscollection areas are defined by radially outwardly extending ribs on thesurface of the screen 20. Thus the area 24a is defined by ribs 26 and 27while the zone 24b is defined by ribs 28 and 29.

The wall of screen 20 is formed with arcuately extending rejects outletopenings 30 formed in each of the rejects areas 24. The purpose of theopenings 30 is to permit the rejects, which are collected on the innersurface of the screen, to pass radially outwardly into the respectiveareas 24a or 24b.

The rotor 15 is also provided with a pair of annular ribs 31 and 32which extend radially into close coaction with the inside screeningsurfaces of screening zones 22a and 22b, at the axial bottom of suchzones respectively, for isolating the rejects collection zone area 24afrom the zone 22a, and for isolating the rejects collection area 24bfrom the zone 22b.

The drum rotor 15 is provided with outlets in its cylindrical shell fordirecting the stock suspension to be screened against the screen in thezones 22a, 22b and 22c. This includes outlets 35a that are axiallyaligned with and open at the screen zone 22a, outlets 35b aligned withthe screening zone 22b, and outlets 35c aligned with the screening zone22c.

The accepts flow through the respective screening zones in the directionof the arrow 38 into a common accepts outlet 40. The rejects from thecollection areas 24a and 24b, and the rejects which flow into the space42 above the rotor from the zone 20a, are collected by a common rejectsmanifold 46 to a rejects outlet 48. The pressure screen of FIGS. 1-2thus has a rejects collection area for each of the three screeningzones, so that rejects which collect on the inside screen surface arepromptly removed in a collection area, before a build-up of rejects canoccur to impede the optimum operation of the screen.

In the operation of the embodiment of the pressure screen, as shown inFIGS. 1 and 2, a stock suspension is applied under pressure to the inlet12 for flowing upwardly in the direction of the arrows 45 into theinterior of the closed top drum rotor. The suspension exits the interiorof the drum rotor 15 through the openings 35 that are positioned todirect the stock into one of the annular screening zones for coaction byone of the sets of foils 16.

The stock which exits the bottom openings 36c are directed against thelower most zone 22c, and rejects travel upwardly along the insiderejects surface of the cylinder 20 until they reach the lowermostopening 30 and escape into the annular rejects collection area 24b. Therib 32 prevents these rejects from mixing with or traveling into thereject surface of the intermediate screen zone 22b.

In a like manner, the rejects which are collected on the inside surfaceof the screen at the intermediate zone 22b travel upwardly until theyreach the outlet 30 positioned between the ribs 26 and 27, at whichpoint these rejects may escape into the rejects collection area 24a forcollection to the rejects outlet 48. The rotor rib 31 prevents theserejects from mixing or contaminating the rejects which are accumulatingat the uppermost screening zone 20a.

The rejects from the upper zone 20a travel upwardly until they escapeinto the enclosed housing 42, for collection with the rejects from theintermediate areas 24a and 24b, to the rejects outlet 48. The ribs 26,27, 28 and 29 of the screen 20 separate the accepts sides of therespective zones from the intermediate rejects collection areas.

By dividing the screen cylinder 20 into a plurality of screening zones,with one or more intermediate rejects collection zones, the extent towhich the rejects must move along the inner inlet surface of the screensections is reduced and therefore the contamination build-up andconsistency build-up is reduced, permitting the screen sections at therespective zones to operate at optimum efficiency.

The closed pressure screen 50 shown in FIG. 3 illustrates a furtherembodiment of the invention. A housing 52 has an inlet 53 for stocksuspension, the stock suspension is carried upwardly within a domeddrum-type rotor 60. The screen means in this embodiment comprises a pairof individual identical annular screens 62 and 64 stacked one above theother and defining corresponding axially spaced screening zones. Theupper screen 62 is separated from the lower screen 64 by a series ofvertical bolts and spacers 66 and which define therebetween an annularrejects collection area or chamber 68.

The rotor 60 is shown as having a shell or wall 70 on which are mountedindividual foils 72 on radially extending arms 75 attached to the wall70. The rotor is formed with an inner concentric tube 78 which ismounted on a spindle 80 for rotation by a belt about the pulley 81. Thewall 70 is provided with stock feed openings 90 and 91 through which thestock suspension is fed respectively to the zones represented by thescreens 62 and 64.

The upper end of the rotor 60 is closed by a domed cap or cover 94 whichis open at the center to discharge into a lightweight reject outlet 95,in which lightweight rejects may be separated from the suspension beforescreening. Outlets 100 and 102 in the housing 50 open into the acceptschambers 105 and 106, respectively for the cylinders screens 62 and 64.

The rotor shell or wall 70 further supports radially extending ribswhich define and isolate the screening chambers from each other,including lower rib 110 and an intermediate rib 112. The intermediaterib 112 separates and isolates the annular rejects collection chamber 68from the screening zone defined by the upper screen 62. The lower rib110 closes off the screening zone of the lower screen from the feed frominlet 53.

The space above the rotor dome 94 forms a further rejects chamber 120.The rejects collection chamber 68 is provided with a rejects outlet 122while the chamber 120 is provided with a rejects outlet 124.

In the operation of the screen as illustrated in FIG. 3, a suspension ofpapermaking stock to be screened is brought up through the inlet 53 intothe interior of the rotor 60, and the lightweight contaminants may beremoved through the top outlet 95. The drum wall openings 90 and 91direct the stock suspension into one or the other of the screening zonesdefined by the screens, to be acted upon by the respective foils 72.

Rejects which collect on the inside or inner reject surface of the lowerscreen 64 move upwardly until they clear the top of the screen 64 atwhich point they move outwardly around the spacer studs 66 into theannular collection chamber 68, for discharge through the outlet 122. Therejects which accumulate on the upper screen 62 are carried upwardlyinto the upper rejects chamber 120 for discharge through the outlet 124.

The operation of the pressure screens according to this invention may berendered even more efficient by providing the rotor 15 or 60 with rotorshells in accordance with the illustrations of FIGS. 4 or 5. FIG. 4shows the drum shell 70 of FIG. 3 particularly adapted for deliveringthe stock suspension into two axially spaced apart screening zones andproviding for a division of stock flow through the shell 70 to therespective top outlet 90 and bottom outlet 91. It will be understoodthat the shell is shown, for the sake of clarity, without the impulsefoils 72 and arms 75 attached, and with the cover 94 removed.

The interior of the drum shell 70 is divided into quadrants by two pairsof generally radially extending vanes including a first set of vanes 200which extend from the internal support tube 78 to the inside wall of theshell throughout the substantial length of the height of the shell. Asecond pair of radially extending walls 202 are positioned in orthogonalrelation to the walls 100 and extend approximately one-half the heightof the wall 70. The result is that there is provided, for the passage ofthe stock suspension, a quadrant of interior space between one of thewalls 200 and one of the walls 202 within which is positioned one of thetop or bottom stock suspension outlets. In this manner, an even flow ofstock suspension is assured for each of the stock outlets 90,91 formedin the rotor shell. At the same time, the vanes formed by the walls 200and 202 impart a rotational movement to the stock suspension within theshell thereby enhancing centrifugal separation of lightweightcontaminants which will flow upwardly into the domed top 94 and throughthe central lightweight contaminant outlet 95.

The rotor embodiment illustrated in FIG. 5 is one which is particularlydesigned for a pressure screen which has three pressure screening zones,such as that illustrated for the screen in FIGS. 1 and 2 above. Therotor shell 215, which may have a closed top or which may be domed forextraction of lightweight contaminants, as illustrated in FIG. 3, isprovided with a central support core 220 from which are mounted threeequally spaced radial ribs or walls 225 defining three arcuate spaceswithin the interior of the shells 15, one each including one of theoutlets 235 corresponding to a separate screening zone. The ribs orwalls 225 also provide the primary function of structural support forthe shell 70 of the drum-type rotor. Also, it will be understood thatthe rotor, at its outer surface, will carry foils either directly on thesurface or on arms 75, as previously described. Again, this rotorconstruction assures a more uniform distribution of the stock suspensionto each of the rotor shell outlets which are positioned in proximity toone of three screening zones for efficient delivery of the stocksuspension to one of the three screening zones.

The invention accordingly provides a pressure screen in whichcontaminant concentration is reduced below that experienced inconventional screens. The separate feeding of the screening zones whichis accomplished by this invention, and the provision for each screeningzone of its own reject zone, reduces the maximum slot or perforationvelocities below that found in conventional screens which have one, orat most, two reject zones. The screening efficiency therefore, acrossthe surface of the screening zones, is improved by the lowering of themaximum velocity of the stock through the openings, whether they beperforations or slots, and by making the velocities more even at thesame time, the concentration of contaminants is reduced, thereby makingit less probable that contaminants will flow through the screenedopenings to the accepts side.

The screen operation may be further optimized by accurately controllingthe flow of the stock suspension to each of the screening zones, whichmay be effected by controlling the relative sizes of the dischargeopenings formed in the rotor shell at the respective zones. Also, whilethe invention is described in terms of embodiments which are bottom fed,it will be apparent to those skilled in the art that the invention isequally applicable to a top fed screen.

What is claimed is:
 1. In a pressure screening apparatus for screening asuspension of papermaking stock having an enclosing housing, acylindrical screen in said housing having an inlet surface and an outletsurface, a drum-type rotor mounted for rotation within said screen andhaving a hollow interior, said rotor having pressure impulse foilsmounted thereon for rotation with said rotor in close proximity to saidinlet surface, a stock inlet in said housing directing a stocksuspension to be screened into the hollow interior of said rotor, astock outlet in said housing for receiving screened fibers passingthrough said screen, a rejects outlet in said housing for receivingreject material from said inlet surface of said screen, the improvementcomprising said screen being formed with at least two axially-spacedapart annular screening zones, said housing with said screen forming twoannular rejects collection areas, one for each of said screening zones,one of said rejects collection areas positioned in the space betweensaid screening zones and arranged to receive rejects from one of saidannular screening zones, and the other of said rejects collection areasbeing positioned at an axial end of said screen and arranged to receiverejects from the other of said annular screening zones, said rotorhaving formed thereon at least two stock suspension outlet openingspositioned respectively to apply such suspension to the inlet surfacesof one each of said annular screening zones, and said rejects collectionareas opening to said rejects outlet.
 2. A pressure screening apparatusaccording to claim 1 in which said drum-type rotor is formed with agenerally cylindrical shell, and radial walls in said shell dividingsaid shell interior into separate flow passages for directing such stocksuspension into discrete ones of the said stock suspension outletopenings.
 3. A pressure screening apparatus according to claim 1 inwhich said cylindrical screen comprises a pair of cylindrical screensections mounted in axially aligned spaced apart relation and each ofsaid screen sections forming one of said annular screening zones, and inwhich said one rejects collection area is formed in the axial spacebetween said screen sections.
 4. A pressure screening apparatus inaccordance with claim 1 in which said axially spaced-apart annularscreening zones are defined by circumferential ribs formed on saiddrum-type rotor guiding said stock suspension from said suspensionoutlet openings into coaction with said screen inlet surface at saidscreening zones.
 5. A pressure screening apparatus according to claim 4in which said cylindrical screen is formed with an opening positionedaxially between said annular screening zones and communicating with saidone rejects collection area for passage of rejects to said rejectsoutlet.
 6. A pressure screening apparatus according to claim 1 in whichsome of said rotor suspension outlet openings are arcuately and axiallyspaced from other of said rotor outlet openings, and in which said rotoris formed with radially extending walls therein forming axial flowpassages for such stock suspension, said flow passages leading todiscrete ones of said outlet openings for providing a uniformity of theflow of stock suspension from said inlet to said screening zones.